Random GPU Activity Decline

I am experiencing an odd behavior with one of my mining setups. I am running a single Sapphire HD7970 in this machine with Windows 10 and Radeon Crimson drivers v17.4.2, and am getting an inconsistent hash rate. I am To clarify this further; immediately after a computer restart and initiating Claymore miner v12.4 i can obtain somewhere in the region of 270H/s to 300H/s. After an inconsistent period of time that ranges from approximately 30mins to 1h30mins the rate will suddenly plummet to about 180H/s. From what i can see this appears to be due to the activity of the GPU (please see screenshot) - it is visible where the card is idle till Claymore miner is started (roughly 98%) and then shortly after it tails off to about 78%. It appears that it is irrelevant whether the card is overclocked or not because the issue happens regardless.

What i can’t work out is what is triggering this change. Has anyone else experienced this issue and/or does anyone have a fix for it?
Thanks in advance.

my suggestion would be to drop your driver to either 16.3.2 or 15.12

Thank you for the suggestion - i will locate these drivers and try that out. Hopefully they are still on AMD's website.

They are... just to the manual search, when you see the screen for the newest driver look on the right column for previous drivers

Thanks for the pointer on this. Strangely at the moment the issue seems to have disappeared and it appears to be related Mozilla Firefox. I have to use the computer for admin related stuff and when running Firefox this issue was occurring all the time. However i have swapped to Opera for browsing and this seems to have resolved the issue. If it reoccurs i will update the post. Thanks for the suggestions CitricAcid.